Mammalian sarcoma viruses are recombinants between helper leukemia viruses and endogenous defective viral sequences. The current project is directed toward understanding the organization of the virus genome and delineating the physical recombination map by two dimensional fingerprints of the RNase T1 resistant oligoribonucleotides of genome RNAs. The physical gene maps of Kirsten and Harvey sarcoma viruses have been deduced. The location of env gene and gene order of Moloney murine leukemia virus and its MCF-inducing strain have been determined.